Eosinophilic Myositis

Eosinophilic myositis (EM) constitutes a group of rare, clinically, and pathologically heterogeneous disorders characterized by eosinophil infiltration of skeletal muscle. Most cases have no known etiological factor and are considered idiopathic. EM should be distinguished from dermatomyositis and other systemic diseases, such as eosinophilic granulomatosis with polyangiitis or hypereosinophilic syndrome. Idiopathic EM can be classified into three subtypes: focal EM, eosinophilic polymyositis, and eosinophilic perimyositis. Recently, a refined classification delineated four phenotypes, including eosinophilic myofasciitis and the classical form of eosinophilic fasciitis, considering these disorders a clinical-pathological continuum from the muscle to the fascia and skin. Pathological studies are mandatory to differentiate and diagnose these entities.